Nonvolatile memory retains stored data even when power is not supplied to the memory. Nonvolatile memory devices are currently widely employed in computers, mobile communication terminals, memory cards, and the like. There are various types of nonvolatile memories, such as, for example, flash memory. Generally, flash memory includes memory cells typically having a stacked gate structure. The stacked gate structure may include a tunnel oxide layer, a floating gate, an inter-gate dielectric layer, and a control gate electrode, which are sequentially stacked on a channel region. Flash memory has its limitations, however, such as relatively low write speed and write/erase degradation.
More recently, new nonvolatile memory devices, such as a resistive random access memory (RRAM), have been proposed. A unit cell of the RRAM typically includes a data storage element that has two terminals and a variable resistive material layer interposed between the two terminals. The variable resistive material layer, commonly referred to as a data storage material layer, has a reversible variable resistance according to whether a low resistive conductive path is formed through the resistive material layer by the electrical signal (voltage or current) applied between the terminals. The applied voltage or current causes the resistive material layer to form microscopic conductive paths called filaments. The filaments appear as a result of various phenomena such as metal ion migration or physical defects. Once a filament forms, it can be broken by the application of a voltage with opposite polarity. The controlled formation and destruction of filaments in large numbers allows for storage of digital data. Resistance changes in the resistive material layer can be sensed to indicate the logic state of the unit cell. While RRAM appears to be a promising nonvolatile memory, there are a number of challenges associated with RRAM. One illustrative challenge is that the distribution cell resistances obtained after writing an RRAM memory array appear to be wide, which results in a less reliable definition of the on and off state. Another limitation concerns scaling of RRAM cells.
Moreover, copper and silver ions are typically used in nanoionic memory because they are stable under atmospheric conditions. One drawback, however, is that these ions are large and slow compared to ions from columns 1A and 1B of the Periodic Table. For example, a high mobility for silver is 10−10 centimeters squared per volt-second (cm2/Vs) in a silver-germanium-sulfur compound, as described more fully by R. Waser, R. Dittmann, G. Staikov, and K. Szot in “Redox-based resistive switching memories—nanoionic mechanisms, prospects, and challenges,” Adv. Mat., vol. 21 (July 2009), which is incorporated herein by reference in its entirety. As a further example, these ions have relatively slow writing pulse widths, as described more fully by M. Tada, T. Sakamoto, K. Okamoto, M. Miyamura, N. Banno, Y. Katoh, S. Ishida, N. Iguchi, N. Sakimura, and H. Hada in “Polymer solid-electrolyte (PSE) switch embedded in 90 nm CMOS with forming-free and 10 nsec programming for low power, nonvolatile programmable logic (NPL),” IEDM (2010), which is incorporated herein by reference in its entirety. For instance, copper has a writing pulse width of ten nanoseconds (ns). This in turn limits such devices to low power programmable logic applications.
As the development of RRAM and other memory devices progress, ions are being utilized more frequently to dope transistor channels. For instance, ions can be drifted towards the surface of graphene in a transistor channel to induce sheet carrier densities in graphene as high as 4×1014 charge carriers per square centimeter (/cm2), for both electrons and holes, as described more fully by D. K. Efetov and P. Kim in “Controlling electron-phonon interactions in graphene at ultrahigh carrier densities,” Phys. Rev. Lett. 105, 246805 (2010), which is incorporated herein by reference in its entirety. Such high charge carrier densities can be achieved because the ions can get closer to graphene than would otherwise be feasible using a gate dielectric.